1. Field of the Invention
The present invention relates to a drive circuit for driving an IGBT (Insulated Gate Bipolar Transistor) device which is a semiconductor switching device and, more particularly, to the protection of an IGBT from an excess current.
2. Description of the Background Art
An IGBT device is characterized by high breakdown voltage and large current capacity similar to a bipolar transistor. Further, similar to a power MOSFET, the IGBT device has a high input impedance due to its insulation gate structure, so that it is also characterized by easiness in driving and rapid switching possibility. Hence, the IGBT device has recently attracted attention as a new device having such characteristics. Devices named IGT, COMFET, GEMFET, MBT or BiFET are commercially available.
FIG. 1A is a diagram showing a symbol of an IGBT device, including a gate G, a collector C and an emitter E. FIG. 1B is a circuit diagram showing an equivalent circuit of the IGBT device. Referring to FIG. 1B, the IGBT device has its gate G connected to a gate of a MOS transistor Q.sub.MOS and its emitter E connected to a source of the MOS transistor Q.sub.MOS. A pnp transistor Q.sub.1 and an npn transistor Q.sub.2 which make up a thyristor are inserted between the collector C and the emitter E. The transistor Q.sub.1 has its emitter connected to the collector C of the IGBT device and its base connected to a collector of the transistor Q.sub.2 and its collector connected to a base of the transistor Q.sub.2. The transistor Q.sub.2 has its collector connected to a drain of the MOS transistor Q.sub.MOS through a modulation resistance R.sub.M and its emitter connected to the emitter E of the IGBT device. A resistance R.sub.BE is connected between the base and emitter of the transistor Q.sub.2. A collector current I.sub.C is shown in FIGS. 1A and 1B.
Now, protecting the IGBT device from an excess current will be described. When the collector current I.sub.C rises to be above a predetermined value, a parasitic thyristor composed of the transistors Q.sub.1, Q.sub.2 turns on. Once current begins to flow in response to this turn on, this current can not be broken and generates heat which destructs the IGBT device. This phenomenon is called latch-up, and the current value is referred to as a latch-up current. Sufficient care must be taken in using the IGBT device with regard to the above problems.
For protecting the IGBT device from an excess current, it is important to restrain an excess current below the latch-up current in the IGBT device. The collector current I.sub.C of the IGBT device depends upon a gate voltage. If the gate voltage is above a predetermined value, the collector current would rise above the latch-up current to cause the latch-up.
FIG. 2 is a circuit diagram showing a conventional drive circuit for the IGBT device. A switching unit SU is provided with voltage sources V.sub.GE1 and V.sub.GE2. A negative potential terminal of the voltage source V.sub.GE1 and a positive potential terminal of the voltage source V.sub.GE2 are grounded. The positive potential terminal of the voltage source V.sub.GE1 is connected to an output point OUT via an output resistance R.sub.O and a switching element S.sub.1. The negative potential terminal of the voltage source V.sub.GE2 is connected to the output point OUT via a switching element S.sub.2.
The output point OUT is connected to a gate of an IGBT device Q.sub.O via a gate resistance R.sub.G. The IGBT device Q.sub.O has its emitter grounded and its collector connected to a positive potential terminal of supply voltage V.sub.CC via a load LD and a current sensor CS. A negative potential terminal of the supply voltage V.sub.CC is grounded. Capacities C.sub.GC and C.sub.GE exist between the gate and collector and between the gate and emitter as a parasitic capacity, respectively.
An output of the current sensor CS is applied to a control system. When the current sensor CS senses an excess current, the control system SY performs its protecting operation to turn off the switching element S.sub.1 and to turn on the switching element S.sub.2, whereby the IGBT device Q.sub.O is forced to be in the OFF condition.
Now, an operation of the above prior art will be described. A control signal form the control system SY decides the ON/OFF condition of the switching elements S.sub.1, S.sub.2. When the switching element S.sub.1 is in the ON condition, and the switching element S.sub.2 is in the OFF condition, the IGBT device Q.sub.O turns on to supply current to the load LD. When the switching element S.sub.1 is in the OFF condition, and the switching element S.sub.2 is in the ON condition, the IGBT device Q.sub.O turns off not to supply current to the load LD. Thus, the current supplied to the load LD is controlled by controlling the ON/OFF condition of the switching elements S.sub.1, S.sub.2.
An abnormal operation with regard to the prior art will now be described. When a motor, for example, is employed as the load LD, the supply voltage V.sub.CC could be directly applied to the collector of the IGBT device Q.sub.O due to an abnormal operation of the motor, as shown by a dotted line in FIG. 2. Since a voltage of several hundreds volt is used as the supply voltage V.sub.CC, the collector current I.sub.C is so rapidly increased as to cause the IGBT device Q.sub.O to generate heat. The collector current I.sub.C is further increased as the temperature of the IGBT device Q.sub.O goes up, so that the parasitic thyristor composed of the transistors Q.sub.1, Q.sub.2 shown in FIG. 2B can easily turn on. As a result, when the load becomes short circuited, latching-up due to a heat generation is liable to occur before the above-mentioned protection operation is completed by the control system SY.
Further, when the load is short-circuited, high voltage from the supply voltage V.sub.CC is instantaneously applied across the collector and emitter. At this time, the gate voltage is instantaneously increased because the capacities C.sub.GC, C.sub.GE exist. The increased voltage .DELTA. V.sub.GE is given by the following equation with an increased voltage .DELTA. V.sub.CE applied across the collector and emitter in step manner: EQU .DELTA.V.sub.GE =.DELTA.V.sub.CE .times.C.sub.GC /C.sub.GE ( 1)
The increased voltage .DELTA. V.sub.CE ranges approximately from tens of volts to hundreds of volts, and the capacity ratio C.sub.GC /C.sub.GE is approximately 0.01-0.05. For example, providing .DELTA. V.sub.CE =100 V and C.sub.GC /C.sub.GE =0.05, .DELTA. V.sub.GE =5 V in accordance with the equation (1). In the case, the gate voltage is increased by 5 V compared with the gate voltage at the normal operation, so that the collector current I.sub.C increases and latching-up may instantaneously occur.
Reducing a value of the gate resistance R.sub.G relieves the latch-up due to the voltage increase of the capacitive component to some extent. However, when the gate resistance R.sub.G is reduced too much, a variation dV/dt becomes large on switching to the voltage source V.sub.GE2, so that the IGBT device Q.sub.O is liable to be latched up. Accordingly, tens of ohms to hundreds of ohms of resistance is required for the gate resistance R.sub.G. It is generally known that when the resistance is increased too much, a switching time becomes long and power loss at the time of switching is increased, so that switching at a high frequency becomes difficult.
In the conventional drive circuit, in order to avoid a latch-up as described above, the gate voltage in the normal operation is reduced by using a low voltage for the voltage source V.sub.GE1. In this way, the amount of a normal collector current I.sub.C is kept small in the IGBT device Q.sub.O, and therefore latch-up does not occur even if the collector current I.sub.C is increased by the short-circuited load. However, increase in ON state resistance of the IGBT device Q.sub.O aggravates power loss in circuit elements other than the load LD.
A drive circuit for a conventional IGBT device structured as has been described drives the IGBT device Q.sub.O with the relatively low gate voltage not to latch-up the IGBT device Q.sub.O. Therefore, such a drive circuit has disadvantages that the ON state resistance of the IGBT devices Q.sub.O is high, and significant power loss is caused in the IGBT device Q.sub.O.